Arrays of so-called second surface mirrors or optical solar reflectors are commonly used on spacecraft for thermal control purposes. A typical second surface mirror for such an array consists of a 2 to 21/2 inch square "window" of quartz or borosilicate glass 6 to 8 mil in thickness. The inner surface of this window is metalized with a silver film or the like having a thickness on the order of 100 to 1000 angstroms. The several second surface mirrors of such a second surface mirror array are bonded by silicone adhesive or the like to the exterior of the spacecraft which provides a supporting substrate for the mirrors. The thickness of the adhesive layer bonding each mirror to the spacecraft is on the order of 3 mil. A spacing on the order of 10 mil is provided between the adjacent second surface mirrors in order to accommodate thermal expansion and contraction.
The second surface mirror arrays in current use are subject to one inherent problem to which this invention is addressed. The problem referred to resides in the fact that the second surface mirrors constitute exterior dielectric surfaces of very high resistivity which may accumulate high electrical potential, on the order of kilovolts in magnitude, and high stored electrical energy, on the order of joules. Eventually, the accumulated electrical charges on the second surface mirrors may become so high as to cause an arc discharge from the mirror surfaces to the underlying metallic skin of the spacecraft. It has been determined that this energy discharge may occur in an arc lasting for about a microsecond with peak currents on the order of 1000 amperes. Such high current arc discharges constitute an extreme hazard to most spacecraft subsystems and hence present a serious problem to the spacecraft.
In connection with this arc discharge problem, it is significant to note that arc discharges from spacecraft exterior dielectric surfaces, such as second surface mirrors, were originally thought to be localized in extent to a few square centimeters or less. Recent experiments have demonstrated, however, that this is not the case. Such experiments have shown, for example, that many thousands of square centimeters of spacecraft surface may discharge in a single arc. Although the discharge duration increases somewhat with area, the peak discharge current also increases with area. Thus, the larger the contiguous areas subject to such arc discharges the greater are the peak currents and hence the potential hazard to the spacecraft. It should also be noted that arc discharges of the kind under discussion are not limited to discharges between individual mirrors and their supporting substrate. Thus, it has been observed that such discharges easily bridge the gaps between the adjacent mirrors.
The arc discharge problem to which this invention is addressed is not limited to spacecraft second surface mirror arrays and may exist, as well, in spacecraft solar arrays. Accordingly, while the invention will be described primarily in the context of a spacecraft second surface mirror array, the invention is not so limited. In this regard, it is significant to note at the outset that, in the present description, the expression "dielectric element" is used as a generic descriptor to include second surface mirrors, and other similar spacecraft mounted dielectric elements.